‘Renaissance gland’: immune influences on lactation and infant protection

During lactation, the mammary tissue becomes critical for infant health. Breastmilk is the gold standard of infant nutrition, containing nutritional, immunological and other bioactive components that direct and contribute to infant immune and microbial development. Additionally, components from the maternal immune system become responsible for protecting and directing the development of the infant after birth through milk. As human milk provides a combination of nutrition and protection, the mammary tissue becomes the ‘renaissance gland’ of the body, capable of performing unique functions. In this review, we discuss the immune system’s role in creating the mammary tissue’s immune environment, the critical immunoglobulin responses underlying passive immunity conferred in breastmilk and additional milk immune components simultaneously offering protection. Immune cells, such as macrophages and T cells, are critical for mammary tissue function during pregnancy and lactation, directing cellular differentiation and lactogenesis. Breastmilk immunoglobulins, such as secretory IgA (sIgA), are critical for protection and microbiota development but are sensitive to maternal environmental exposures such as antibiotics and health state. Importantly, immunoglobulin’s protective capabilities are supported by other immune components, including milk-derived leucocytes, lactoferrin, human milk oligosaccharides and the milk’s microbiome. Ultimately, the mammary tissue and, by extension, breastmilk ensure vertical transfer of maternal immune responses, representing the culmination of maternal exposures and an important potential avenue for protecting infants through harnessing maternal immunity.